1. Field of the Invention
The present invention generally relates to a power supply circuit and, more particularly, to a power supply circuit for intermittently actuating a filament of a vacuum fluorescent display.
2. Description of Related Art
A vacuum fluorescent display (VFD) has been widely applied to various products such as phonographic equipment, video recorders, CD (compact disc) players and VCD (video compact disc) players. An isolated power supply is usually used for a filament of a vacuum fluorescent display; otherwise, a malfunction may occur in the vacuum fluorescent display.
In general, a switching power supply is used to provide an isolated power for a filament of a vacuum fluorescent display, and provides other power supplies for use in the vacuum fluorescent display.
As shown in FIG. 2, a conventional switching power supply for a VCD player (not shown) provides a set of various DC (direct current) voltage signals for use in the VCD player. As the switching power supply is conventional technology, thus a detailed description thereof is omitted here.
In FIG. 2, the ends (VF+, VF-) of a filament of a vacuum fluorescent display FT1 are connected with an inverter 50 which is comprised of a transformer 51 and two transistors 52 and 53. The inverter 50 receives a DC voltage (+5 volts) from the switching power supply, generates a high-frequency oscillation therein, and then sends an AC (alternate current) voltage (with an amplitude of five volts) from a secondary winding of the transformer 51 to the filament of the vacuum fluorescent display. Therefore, an isolated power supply can be provided for the filament of the vacuum fluorescent display by using the inverter 50.
In addition, a signal MPO in FIG. 2 is a control signal for controlling the power supply of the filament. The control signal MPO can intermittently actuate the filament at a high frequency by using a plurality of transistors, thereby to prolong the lifetime of the vacuum fluorescent display.
However, the above-described switching power supply using the inverter 50, which provides an isolated power supply for a filament of a vacuum fluorescent display, has a complicated circuit structure. Therefore, there is a need to contrive a power supply circuit with a more simplified structure which can provide the filament of a vacuum fluorescent display with an isolated power supply.